Positron Emission Tomography (PET) is one of the nuclear medical inspection methods which measure the existence and position of the radioactivity. While the X-Ray CT measures the transmission of radiation by arranging radiation source out of analyte (human body), the PET measures the place of the radiation isotope (RI) distributed in specific internal organs within the human body by administrating radiopharmaceuticals and measuring emitted radiation. The PET utilizes the radiopharmaceuticals tagged by the RI, which releases positrons, as the radiation source. The positron annihilates by combining with electron bound near, and two strong gamma rays having strong transmission power fly in opposite directions from that point alternatively. By simultaneously counting a pair of radiations by detectors arranged around the human body, direction and location of the radiation source can be detected. Pet is a technique to reconstruct three dimensional density of the radiation source inside where the data were counted at the same time, for example cancer and general examination, brain, organs and various diagnostic like cardiac function, brain and nervous system research, and has been used inn pharmacokinetics and metabolism research.
Semiconductor elements are used as radiation detecting element for PET, compared with radiation detecting elements for scintillator and a photomultiplier tube that was conventionally used, it is small, lightweight, high image resolution characterized by direct radiation converted to electrical signal, in recent year research has been developed.
FIG. 7 (a) is a cross-sectional view of a radiation detecting element using a conventional Schottky diode disclosed in Patent Document 1. On the one surface of the CdTe-type semiconductor substrate 102, electrode 101 comprised from InCdTe is formed. In the interface of the electrode 101 and the semiconductor substrate 102, a Schottky junction is formed. On the other side of the semiconductor substrate 102, the ohmic electrode 103 comprised from Pt is formed. When the bias voltage is applied so that the high potential electrode 101 to the semiconductor substrate 102, a Schottky junction becomes reverse biased. In this case, electron-hole pairs are generated in the depletion layer in the Schottky junction and the radiation incident on the semiconductor substrate 102 to move within the semiconductor substrate 102 by an electric field formed by the bias voltage. Incident radiation can be detected at high speed by measuring the current flowing between electrode 101 and the ohmic electrode 103.